


Undersun

by FreddyFazbutt



Category: Undertale (Video Game)
Genre: Body Horror, Both viruses eventually corrupt their host, Chara Is Their Own Warning, Chara hates Frisk, Chara is a manifestation of Nightbite/Sunbite in it's purest form, Do not hug Frisk they will INFECT you, Evil Chara, Flowey Is A Dick, Fluff and Angst, Frisk Needs A Hug, Frisk bleeds determination, Frisk gets worse as time goes on, Gen, Genocide is known as Plague, Humans get Nightbite instead, It blackens them, It bleaches their color scheme, Main Mechanics of Undertale take a backseat, Monsters are slowly dying, Monsters can get Nightbite, Nightbite affects the soul that is why Monsters die from it, POV Frisk, POV Third Person, Pacifist is known as True Cure, Poor Frisk, Poor Toriel, Rape is a common theme for Frisk's nightmares, Sickness, The monsters have a virus, This is pretty fucking sad and dark, Toriel is falling down, Toriel needs a hug, and increases determination to unsafe levels, and it can lead to falling down, and lowers their defense while increasing attack, at first, but it slowly kills them, it's called Sunbite, to the CURE and INFECT options, which can create rips in time and space
Language: English
Status: In-Progress
Published: 2017-06-10
Updated: 2017-11-11
Packaged: 2018-11-12 08:03:08
Rating: Mature
Warnings: Graphic Depictions Of Violence, Major Character Death, Rape/Non-Con
Chapters: 5
Words: 2,494
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11157675
Author URL: https://archiveofourown.org/users/FreddyFazbutt/pseuds/FreddyFazbutt
Summary: An au where a deadly virus named Sunbite ravages the underground it bleaches monsters whole look makes their attacks stronger while leaving their defense low and eventually leads to corruption of the body and Falling down however there is a prophecy that states: One day one who carries a great burden who has seen the surface will fall to the underground and cure the monsters from their torment. That prophecy refers to you Frisk a human child infected with the human equal of Sunbite Nightbite however monsters can get this disease as well through a single touch from a human on the other hand Sunbite can be cured if the human touches the soul of the monster infected with Sunbite.





	1. The Plague begins

**Author's Note:**

> A dark story with sad stuff like angst and maybe a bit of fluff

**Notes for the Chapter:**

> This is Undersun Frisk if you couldn't tell

PAIN

SO MUCH PAIN

"HELP ME!" but nobody came.........

Another scream.............

"SOMEONE PLEASE HELP ME!" but nobody came......

A child opened their eyes to find themselves standing on a sick and dying patch of flowers which made the child sad. The flowers looked horrible and sad the child or Frisk as they are known just stood there as they took in the sad broken flowers made worse by their fall.

Frisk coughed and scratched at their chest which was a dark blue and leaking blood or so that what they thought it was. A year ago they had been normal an average kid with average problems until that faithful day when their skin and clothes turned blue and they found red liquid leaking from their eyes and mouth which made their parents try to do everything to fix their child but Frisk could not be treated for no doctor could identify this strange illness that had taken Frisk like the plague.

Frisk found they couldn't go to bed without having terrible nightmares of a faceless man injecting them with a mysterious blue liquid and torturing them in various ways like forcing them to watch their parents being raped or worse being forced as their parents were torturing them in numerous ways.

That wasn't the only thing either Frisk also started hallucinating a dark black figure with red eyes that would stalk them from the corner of their vision and disappear when Frisk looked at them directly. Sometimes Frisk would bleed the strange red blood like fluid from their crevices which was anywhere something could come out of.

They also got this piercing pain in their chest like something was seriously wrong but it went away after a while but still it hurt a lot and Frisk's parents tried their hardest but in the end they stopped caring about Frisk. They had lost hope and one day Frisk just left unable to deal with broken parents anymore taking only their stuff and a picture of better times.

That was how they ended up here but now that they thought of their parents they began to cry however this time a dark laugh responded back "HEHEEHEHE LOOK AT YOU YOU'RE SO PATHETIC" Frisk stopped crying and looked around for the voice they were hearing and their heart stopped. Standing in front of them was the dark figure who had a dark smile on their face as they said "Greetings Partner I'm Chara"


	2. Sickness Prevails

**Summary for the Chapter:**

> Chara torments Frisk and forces them to INFECT a monster

**Notes for the Chapter:**

> I know the image isn't that different from the original Chara but I wanted to show how better off Chara is in this Au over everybody else

 

 

Frisk is still frozen as Chara continues to grin that dark grin and then it's gone as they say "YOU STUPID USELESS CHILD YOU ARE GOING TO DO WHAT I SAY OR I WILL TORMENT YOU WITH IMAGES OF YOUR DEAD PARENTS"

Frisk bravely shakes their head and screams in pain when images of their parents burnt alive flash through their head. Frisk willingly submits and Chara smiles as they say "GO NOW" and Frisk begins walking to the door and stops to push the doors open as Chara starts laughing their laughter torturing the poor child whose head is burning with pain from the images.

Frisk enters the room and spots a flower that is bigger though this one looks sicker so Frisk says "Hey Mr Flower do you know where I am?" The flower turns around and sports a horrifying appearance: The face is cracked all over with raw magic seeping out of the cracks also the flower's face is sharp and evil looking with a bright color scheme.

The Flower says "Another stupid child fallen into the underground I'm not even going to pretend for you"  Suddenly what Frisk assumes is their soul appears and it's a ugly red thing with the strange red liquid dripping from it and there is a blue substance starting to creep on the red.

Flowey shoots out seeds that glow white and fires them at Frisk's soul who manages to dodge them but just as Flowey is going to throw more. A fireball knocks the plant away and a goat lady that looks worse off than Frisk appears.

She has half her face gone leaving a flame to surge out and she has no left arm  but she says "I am Toriel caretaker of the Ruins and I would like you to follow me". Chara's voice echoes through Frisk's head "YOU MUST SPREAD THE PLAGUE" as they walk behind Toriel. Frisk is taken off guard by the world going dark and when Chara says "YOU MUST MAKE THIS CREATURE SUFFER OR ELSE"

Frisk is presented with four options: INFECT, CURE, FIGHT and MERCY

Frisk presses INFECT and finds themself reaching out to touch the Froggit that stood before them and watching it lose it's previously bright colors to a dark scheme that wasn't all either the froggit had the red liquid leaking from its eyes.

The world faded back as the froggit was now vomiting the red liquid which made Frisk feel horrible they had given whatever horrible disease they had to this poor monster. The froggit hopped away as Toriel said "Why'd you stop? My child we're barely there" Frisk apologized and began walking again. 


	3. You are infected with Determination

**Summary for the Chapter:**

> Frisk does some puzzles and is afflicted by something else via their disease which leads to Toriel dying

**Notes for the Chapter:**

> Frisk gets worse in this chapter

Frisk was led by Toriel to a puzzle that had spikes blocking the way. Toriel said I labelled the switches for you my child and waited for Frisk to complete the puzzle. Frisk walked until they spotted a switch with writing underneath it and Chara said "PRESS THIS THIS SWITCH MY CHILD -TORIEL"

So Frisk pressed it and the spikes popped down. Toriel smiled and clapped to show she was proud then led Frisk into the next room which looked complicated but Toriel deciding it was too tough went to touch Frisk's hand but Frisk flinched back. They remembered what had happened to the Froggit when they had touched it however Toriel looked a little upset so Frisk signed "I have a bit of trust issues"

Toriel didn't look like she believed them but she led it slide and decided to show the pattern instead. She did the pattern until Frisk got it and they continued but then Frisk fell to the ground as their mouth and eyes began leaking excessive amounts of red liquid.

Toriel said "Are you okay my child?" But Frisk didn't respond and instead stood up with a nasty grin on their face with their face still bleeding red liquid Frisk said in a distorted voice "T H E  U N D E R G R O U N D  W I L L D I E  A N D  Y O U  W I L L  B E  T H E  F I R S T"

They slammed their hand into Toriel's chest causing red liquid to surge from her entire body and drain her of her bright colors. Toriel fell to her knees in great pain as her body began to melt into a puddle. Frisk having snapped out of it gasped and tried to save Toriel but it was too late Chara laughed as they said "GREAT JOB PARTNER"

Frisk gulped as they looked down at their hands which were now slightly glowing red and they felt the red liquid continue to fall from their eyes. They felt disgusting and sicker then they had been but deciding to take Toriel's robe to remind themselves of the mom that never was.

The rest of the journey felt wrong like Toriel was supposed to guide them through the whole thing and horrible images of a bloody Toriel still played in their mind and eventually they came upon what they thought was probably Toriel's house or it had been.

Their journey up until that point had them INFECTing every monster and making them bleed the same red liquid which Frisk was starting to enjoy. Frisk started to enjoy making the monsters suffer and by that point had thrown Toriel's robe away like it was trash.

Frisk entered the house and ransacked it for anything they might need and then headed down to the basement where they walked down a long hallway not even noticing the red mist following them and darkening everything in the ruins.


	4. Diseased Snow

**Summary for the Chapter:**

> Frisk meets Sans and the plague moves into Snowdin

**Notes for the Chapter:**

> Beware of determination and enjoy this chapter also enjoy this new image of undersun sans and papyrus

 

Frisk smiled red liquid running down their eyes and mouth as they entered the forest and kept walking until they heard a CRACK and walked faster until they came to a bridge they heard a voice say H U M  A N -

The voice was cut off by a loud coughing coming from behind them and Frisk turned around to see something that would terrify most people.

A skeleton but he had a dark void in his face and glowing blue eyes and his skin was a sickly white. He said in a voice that sounded strained

"Sorry about that anyway I'm Sans Sans the skeleton hey your a human right? thats hilarious i'm a sentry but well you know I don't really care about capturing humans now my brother he's a human hunting FANATIc but don't worry he's harmless anyway go through these bars my bro made the bars too wide to stop anyone"

Frisk went through the bars to what looked like a checkpoint and Sans who had followed them and he said "Quick behind that perfectly placed plant!"

They didn't and instead fell to the ground and started puking up more red liquid when Papyrus came he saw his brother trying to comfort what looked like a human but this creature had a strange red liquid running down their cheeks and their eyes were dark voids which was clearly not human so he went up to them and said.

"CREATURE ARE YOU OKAY THE GREAT PAPYRUS NOTICED YOU WERE EXPLELLING RED LIQUID FROM YOUR BODY"

Frisk looked up at the skeleton who had hall of his body gone but in place of it a blue glowing magic half replaced what was obivously a diseased part of the skeleton "But how Frisk hadn't INFECTED him yet?"

They then remembered Toriel had only one arm and half her face was gone and Frisk felt their stomach plummet as Papyrus with Sans lifted them up and led them away. They walked for many hours sometimes stopping to take breaks and eat but finally reached what looked like a peaceful town.

Papyrus said "AH HERE WE ARE HUMAN THIS IS SNOWDIN YOU WILL BE FINE NOW FOR I THE GREAT PAPYRUS WILL TAKE CARE OF YOU" He then carried Frisk into a medium sized house and set them on the couch and Papyrus left to go make something for the sick human.

Sans then sat down on the floor in front of Frisk who was coughing and he said "Listen human I don't know what you are but you are definitely dangerous I surveyed your soul and despite the fact you have no LOVE you did something to the ruins so I'm only going to say this once and his glowing magic eyes disappeared and he said "Y O U  A R E  G O I N G  T O  H A V E  A  H O R R I B L E  T I M E"  and his eyes reappeared.

Papyrus came back noticing the room's dark atmosphere and said "HUMAN I HAVE MADE YOU A BOWL OF MY DELICIOUS SPAGHETTI" and he set a bowl of spaghetti into Frisk's lap.


	5. The tragic suffering of Papyrus and the price of happiness

**Summary for the Chapter:**

> Sans sits and talks with Frisk who is having second thoughts of INFECTing Papyrus after learning of his suffering we learn the truth of what awaits at the end of True Cure.

**Notes for the Chapter:**

> WARNING very sad and dark will probably make you wish you hadn''t read this chapter.

Frisk stared at Papyrus as they picked up the bowl but not eating it. Papyrus said "IT'S REALLY GOOD TRUST ME HUMAN EVEN THAT LAZYBONES SANS LIKES IT" they turned to Sans who said "Go on try it kid"

They picked up the spoon and began eating it as they found the taste to be gross but not wanting to give Sans a reason to hurt them they ate the whole thing. Papyrus smiled and said "SEE I KNEW IT NOBODY CAN RESIST THE MASTERPIECES OF MASTER PASTA CHEF PAPYRUS NYEHEHE!"

He left the room leaving Sans and Frisk alone as Sans said "Good you actually know how to listen but there's something you need to know bout Paps kid he does not have long for this world" Frisk looked at him confused as they wiped their eyes of the determination before he continued "You see Paps is in stage 3 of the disease and it won't be long before he well you know"

Frisk looked down at their feet as Sans said "Well kid i'm sure you're wondering why I told ya this and that is because I was maybe thinking you would actually understand to please please don't hurt him he's all I have left and if he is dammed to die anyway why bother?"

Frisk had a feeling that if they spared Papyrus then Chara would not be happy and torment them some more but they thought about how nice he had been and decided that they would rather take the torture given by Chara then INFECT him. They opened their mouth and spoke for the first time in a very long time "S-sans I c-can save your brother"

Sans looked rather surprised and said "What do ya mean kid?" They pulled out their SOUL which was a festering thing of dripping determination and said "T-this d-disease I have is called Nightbite and I-it can cure monsters of s-sunbite"

Sans said "Well kid I have a hard time believing that but at this point I'm willing to try anything" He picked Frisk up and led them to Papyrus's room where loud coughing and splashing sounds could heard. Sans opened the door revealing Papyrus coughing up the same red liquid that Frisk's body produced in surplus amounts.

Sans brought Frisk closer as Papyrus's SOUL appeared the red liquid covering it except for a tiny part of it. Frisk touched his SOUL which caused the liquid to fall off the SOUL onto Frisk's SOUL who began coughing as Papyrus was now cured of his disease.

He fell asleep as his body returned to normal while Sans carried Frisk back to bed and said "You saved him kid I don't know how to thank you" Unable to form words the sick child just smiled as the voids in their eyes got bigger and the determination flowed out more as they got sicker but managed to get out "S-S-Sans s-s-show me your SOUL"

He said "No kid don't worry about me you need rest" but Frisk was not taking no for an answer as they reached into Sans chest and forced the red liquid onto their SOUL accepting his illness.

Sans found himself normal again as the kid coughed the voids getting even bigger connecting now as the determination spilled out onto everything like blood as Sans said "Kid no stop you can't do this anymore you saved my bro and for that I thank you but the more you use this on monsters the more you get closer to dying"

Frisk smiled and opened their mouth to say something but sleep overtook them and they faded into their dreams.


End file.
